Photonic crystal fibre with a shortwavelength cutoff

Summary form only given. Conventionally, optical fibres become stronger waveguides as wavelength decreases. The fundamental mode becomes more confined to the core, successive higher-order modes appear, and the V-value rises without limit. Photonic crystal fibres (PCFs) have holes in the cladding that run along the length of the fibre. These give the cladding a lower effective refractive index than the glass it is made from, allowing: a solid core (a missing hole) to guide by total internal reflection. If the core and cladding glasses are identical, the strong wavelength dependence of the cladding effective index gives V an upper bound and allows the fibre to be endlessly single mode.